Currently marginal energy is produced mainly by gas turbine, either in simple cycle or combined cycle configurations. As a result of load demand profile, the gas turbine base systems are cycled up during periods of high demand and cycled down or turned off during periods of low demand. This cycling is typically driven by the Grid operator under a program called active grid control, or AGC. Also, in many electrical markets, peak power demands occur when it is hottest outside. Gas turbines naturally loose power and efficiency at elevated ambient temperatures which further increases the number of gas turbines that must be run during peak periods. Unfortunately, because industrial gas turbines, which represent the majority of installed base, were designed primarily for base load operation, when they are cycled, a severe penalty is associated with the maintenance cost of that particular unit. For example, a gas turbine that is running base load could go through a normal maintenance once every three years, or 24,000 hours at a cost in the 2-3 million U.S. dollar range. That same cost could be incurred in one year for a plant that is forced to start up and shut down every day.
Currently these gas turbine plants can turn down to approximately 50% of their rated capacity. They do this by closing the inlet guide vanes of the compressor, which reduces the air flow to the gas turbine, also driving down fuel flow as a constant fuel air ratio is desired in the combustion process. Maintaining safe compressor operation, and compliance with emissions requirements, typically limit the level of turn down that can be practically achieved.
The safe compressor lower operating limit is improved in current gas turbines by introducing warm air to the inlet of the gas turbine, typically from a mid stage bleed extraction from the compressor. Sometimes, this warm air is also introduced into the inlet to prevent icing. In either case, when this is done, the work that is done to the air by the compressor is sacrificed in the process for the benefit of being able to operate the compressor safely to a lower flow, thus increasing the turn down capability and preventing icing of the inlet. This has a further negative impact on the efficiency of the system as the work performed on the air that is bled off is lost. Additionally, the combustion system also presents a limit to the system.
The combustion system usually limits the amount that the system can be turned down because as less fuel is added, the flame temperature reduces, increasing the amount of carbon monoxide (CO) emissions that are produced. The relationship between flame temperature and CO emissions is exponential with reducing temperature, consequently, as the gas turbine system gets near the flame temperature limit, the CO emissions spike up, so a healthy margin is kept from this limit. This characteristic limits all gas turbine systems to approximately 50% turn down capability, or, for a 100 MW gas turbine, the minimum power, or maximum turn down, that can be achieved is about 50%, or 50 MW. As the gas turbine mass flow is turned down, the compressor and turbine efficiencies fall off as well, causing an increase in heat rate of the gas turbine. Some operators are faced with this situation every day and as a result, as the load demand falls, their gas turbine plants hit their lower operating limit and they have to turn the gas turbines off, which costs them a tremendous maintenance cost penalty.
Another characteristic of a typical gas turbine is that as the ambient temperature increases, the power output from the gas turbine system goes down proportionately due to the linear effect of the reduced density as the temperature of air increases. Power output can be down by more than 10% from nameplate output during hot days, typically when peaking gas turbines are called on most to deliver power.
Another characteristic of typical gas turbines is that air that is compressed and heated in the compressor section of the gas turbine is ducted to different portions of the gas turbine's turbine section where it is used to cool various components. This air is typically called turbine cooling and leakage air (hereinafter “TCLA”), a term that is well known in the art with respect to gas turbines. Although heated from the compression process, TCLA air is still significantly cooler than the turbine temperatures, and thus is effective in cooling those components in the turbine downstream of the compressor. Typically 10% to 15% of the air that comes in the inlet of the compressor bypasses the combustor and is used for this process. Thus, TCLA is a significant penalty to the performance of the gas turbine system.
Another characteristic of many large frame engines used to generate power is that the RPM is fixed because the shaftline of the gas turbine is fixed to the generator and the generator must spin at a specific speed to generate electricity at a specific frequency, for example 3600 RPM for 60 HZ and 3000 RPM for 50 HZ. The term “shaftline” means the shaft of the gas turbine and the shaft of the generator and including any fixed ratio gearbox attached between those shafts, so that at all operating conditions the ratio of revolutions per minute (RPM) of the gas turbine shaft to the RPM of the generator shaft remains constant. In gas turbines that have free turbines or multiple turbine shafts within, this is not true. Consequently only one shaft of a multi-shaft gas turbine, the one tied to, or on the shaftline with, the generator, has to spin at a constant rpm. This is a significant consideration when injecting air upstream of the combustor.
On a multi-shaft engine, like the LM6000 for example, when the air is injected upstream of the combustor, the high pressure turbine actually speeds up which drives the high pressure compressor harder, which in turn induces more air flow through the gas turbine's low pressure compressor, and the compressor as a whole. Therefore, the increased airflow that is being injected upstream of the combustor is the injected air plus the additional flow that is induced in the gas turbine engine's core. Since the low pressure compressor is tied to the low pressure turbine (LPT) and generator, it spins at 3600 RPM (for a 60 HZ generator) and additional air flow goes through the LPT because of the reduced pressure between the high pressure compressor (HPC) and LPC. In other words, since the high pressure compressor is working harder and inducing flow through the low pressure compressor, the low pressure compressor does not need to work as hard to compress the air going to the combustor, so more of the energy that drives the low pressure turbine and the power turbine is available to drive the generator (or other load).